This protocol is to assess the effects of a change in exercise performance at various stages in pregnancy on feto-placental growth rate. It will test the hypothesis that the rate of both placental and fetal growth correlates with exercise performance at various time points in pregnancy. Randomized subjects and controls will either increase, decrease or maintain weekly exercise volume with advancing gestation. Exercise volume is monitored carefully throughout and maternal, placental and fetal morphometrics are obtained every four weeks.